Radioactive materials are often detected and identified by measuring gamma-rays and/or neutrons emitted from the materials. The energy of gamma-rays is specific to that particular material and acts as a “finger print” to identify the material. Similarly, neutron energy is particular to the material, and may be used to identify the material. Of very high value are detectors capable of identifying the distinctive time-correlated signatures corresponding to neutrons and gammas emitted by fissioning material from within a background of uncorrelated natural radiation. A detector capable of distinguishing neutrons from gammas, as well as offering a fast response time typically has better capability for detecting the distinctive time-correlated events indicative of the presence of fissioning nuclei.
The ability to detect gamma rays and/or neutrons is a vital tool for many areas of research. Gamma-ray/neutron detectors allow scientists to study celestial phenomena and diagnose medical diseases, and they have been used to determine the yield in an underground nuclear test. Today, these detectors are important tools for homeland security, helping the nation confront new security challenges. The nuclear non-proliferation mission requires detectors capable of identifying diversion of or smuggling of nuclear materials. Government agencies need detectors for scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events. To better detect and prevent nuclear incidents, the Department of Energy (DOE) and the Department of Homeland Security (DHS) are funding projects to develop a suite of detection systems that can search for radioactive sources in different environments.
One particularly useful type of radiation detection, pulse shape discrimination (PSD), which is exhibited by some organic scintillators, involves subtle physical phenomena which give rise to the delayed luminescence characteristic of neutrons, providing a means of distinguishing neutrons from the preponderance of prompt luminescence arising from background gamma interactions. The mechanism by which this occurs begins with the excitation process which produces excited singlet (S1) and excited triplet (T1) states nonradiatively relaxes to the configuration, as shown in FIG. 1. In FIG. 1, the basic physical processes leading to the delayed fluorescence characteristic of neutron excitation of organics with phenyl groups is shown.
Since the triplet is known to be mobile in some compounds, the energy migrates until the collision of two triplets collide and experience an Auger upconversion process, shown as Equation 1:T1+T1→S0+S1  Equation 1
In Equation 1, T1 is a triplet, S0 is the ground state, and S1 is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, which is represented as Equation 2:S1→S0+hv  Equation 2
In Equation 2, hv is fluorescence, while S0 is the ground state and S1 is a first excited state. The enhanced level of delayed emission for neutrons arises from the short range of the energetic protons produced from neutron collisions (thereby yielding a high concentration of triplets), compared to the longer range of the electrons from the gamma interactions. The resulting higher concentration of triplets from neutrons, compared to gamma interactions, leads to the functionality of PSD. The observation of PSD is believed to be in part related to the benzene ring structure, allowing for the migration of triplet energy.
FIG. 2A shows a typical plot of logarithmic population versus linear time (ns) for stilbene. Population is the singlet excited state population, which is proportional to the output of light from a test crystal under examination, in this case a stilbene crystal, after the crystal it is excited by high energy radiation. As can be seen from the plot, some light is produced by the crystal almost immediately, referred to as prompt luminescence, and other light is produced from the crystal over a period of time, referred to as delayed luminescence. Generally, the plot for each type of radiation will have a steep component 202 and a tail component 204, where the differentiation point 206 between the two is defined in the region where the slope of the line changes dramatically. In this example, the steep component 202, tail component 204, and differentiation point 206 for the Neutron curve is labeled. Note that the steep component, tail component, and differentiation point for the Gamma curve is different for stilbene, and other compounds which possess good PSD properties. Compounds which do not possess good PSD properties will generally not have substantial differences in the curves plotted for Gamma and Neutron radiation. The upper line in the plot shown in FIG. 2A is a Neutron-induced scintillation pulse shape, while the lower line is a Gamma-induced scintillation pulse shape. As can be seen, stilbene is able to differentiate between the Neutron and Gamma pulse shapes, and produces noticeably different luminescence decay lineshapes for each radiation type. However, not every compound has this ability to separate between Gamma and Neutron pulse shapes, and therefore compounds which do are very useful for PSD, as Gamma and Neutron luminescence decay plots have different pulse shapes for these compounds.
Once the population versus time plot has been determined for each test crystal under examination, if it appears that there is PSD for the crystal type, the area (QS) under the tail component of the curve for each type of radiation is calculated, along with the area (QF) under the entire line for each type of radiation. By dividing the total area (QF) into the tail area (QS), a scatter plot of the ratio of charge versus the pulse height can be produced, as shown in FIG. 2B for stilbene. FIG. 2B shows a plot of the ratio of charge (QS/QF) versus the pulse height, which correlates to an output of a light detector, such as a photomultiplier tube. The x-axis represents the pulse height, which is proportional to the energy of the event. Gamma events correspond to light produced by Compton electrons generated in the detector material. Neutron events correspond to proton recoils in the detector material; lower energy proton recoil events correspond to “glancing angle” interactions between the neutron and proton in the detector material, while a high energy “knock-on” interaction between a neutron and a proton will produce a higher energy event.
Referring to FIG. 2B, at hv equal to about 1600V, conventional scintillators utilizing stilbene exhibit a neutron-to-gamma (n°/γ) separation S of about 0.132. The greater the separation S of neutron-to-gamma, the better PSD performance can be expected.
It is with these scatter plots that good PSD separation can be determined, which is defined as PSD separation, S, which is the gap between the mean ratio of charge (QS/QF) for Gamma and the mean ratio of charge (QS/QF) for Neutron taken over an extended period of time. The higher this separation, S, is, the better the compound is at PSD separation.
It is generally accepted in the prior art that stilbene offers good PSD. However, stilbene, generally grown from melt, is difficult to obtain. Therefore, a number of other organic molecules have been examined Unfortunately, most research in this area has concluded that many known liquid and solid materials, including many compounds having benzene rings, do not possess PSD properties comparable to single-crystal stilbene. Despite the difficulty in identifying compounds with suitable PSD properties, the inventors previously succeeded in demonstrating several exemplary compounds with suitable PSD properties and capable of being grown from solution, including 1-1-4-4-tetraphenyl-1-3-butadiene; 2-fluorobiphenyl-4-carboxylic acid; 4-biphenylcarboxylic acid; 9-10-diphenylanthracene; 9-phenylanthracene; 1-3-5-triphenylbenzene; m-terphenyl; bis-MSB; p-terphenyl; diphenylacetylene; 2-5-diphenyoxazole; 4-benzylbiphenyl; biphenyl; 4-methoxybiphenyl; n-phenylanthranilic acid; and 1-4-diphenyl-1-3-butadiene.
Moreover, crystals such as stilbene, generally grown from melt, are difficult to obtain. Therefore, organic liquid scintillator cocktails comprised of an aromatic solvent, such as toluene, a primary and a secondary fluor, have been developed and are commercially available, however, liquid scintillators do not exhibit PSD properties comparable to single-crystal stilbene, and are also hazardous to field, because these compounds typically include flammable, toxic, and otherwise hazardous materials that limit application to sensitive environments such as aviation, military applications, medical applications, and etc. Moreover, the above crystals, especially when grown from solution, tend to be relatively fragile, making safe and efficient transport difficult.
Accordingly, it would be beneficial to provide organic materials comparable to or better than stilbene in relation to PSD properties for neutron radiation detection, but in a form that is easier to fabricate into large monoliths which are durable, and which do not introduce hazardous material into the radiation detection process.